A variety of mechanical parts have the problem of corrosion. For example, the rolling bearing parts for the automotive electrical equipment and engine auxiliaries including alternators, electromagnetic clutches and the like have the problem of corrosion resulting from permeation of water on the road, salt water or sea water during the operation; the rolling bearing parts for industrial machinery including compressors, blowers and the like also have the problem of corrosion resulting from permeation of cooling water and penetration of rainwater or sea water when installed outside; and the rolling bearing parts for the motor of home electrical appliances such as washing machines, refrigerators, and bathroom ventilating fans and bathroom ventilator dryer system which have become popular in recent years also have the problem of corrosion because they are exposed to high humidities.
In the above-mentioned rolling bearings subject to corrosion, the corrosive substances are prevented from invading the bearings by provision of a proper sealing or modifying the mechanism.
For the above-mentioned rolling bearing in the automobile, for example, it is proposed to install the rolling bearing at a position where the rolling bearing does not come in direct contact with muddy water or the like or to mount a fender or the like to protect the rolling bearing.
In light of the mechanism of rolling bearing, however, complete sealing of the rolling bearing cannot be achieved. Then, a lubricating grease used for the rolling bearing is required to have rust preventing characteristics.
For example, JP 2883134 B discloses a grease composition comprising rust inhibitors, i.e., an oil-soluble organic inhibitor, a water-soluble inorganic passivator and a nonionic surfactant.
JP 5-140576 A discloses a grease composition to be enclosed in bearings, comprising barium sulfonate as a rust inhibitor.
Sodium nitrite, which has been considered to be good and widely used as the water-soluble inorganic passivator was found to produce a carcinogen, N-nitroso-amine when reacting with secondary amine. There is serious concern about an adverse effect on human body.
The organic sulfonate such as barium sulfonate has been widely used as the oil-soluble organic inhibitor. However, when the organic sulfonate is used alone, the rust inhibiting effect is found to be unsatisfactory under severe conditions, especially in the presence of salt water or seawater.
In addition, there is a movement to impose self-restraint on barium sulfonate as an environmentally damaging substance, as described in NTN Technical Review No. 73 (2005), pp 10-13.